SimpleDB provides simplicity and infinite scalability (relatively), but that comes with some big compromises – the biggest being eventual consistency and no transactional integrity. Eventual consistency means data updates are not reflected immediately – they propagate over time (usually under 5 mins), which can create some unique challenges for transactional applications. Without transactional integrity, you can’t be guaranteed that a set of related updates are applied together, which creates the risk of data corruption.

RDS, on the other hand, provides all the advantages of a traditional relational database (MySQL, specifically), but comes with the cost of complexity and scalability. Amazon does reduce a significant amount of the complexity and scalability issues with RDS. They provide all the generic database administration services, including backups. And they provide the ability to scale both CPU and storage capacity with simple API calls. But there is a limit to how high an RDS instance can scale, at which point you have to manually resort to horizontal scaling techniques like clustering and partitioning – which are not automatically supported by RDS. While both RDS and SimpleDB have limitations, used together they offer a very powerful and flexible solution.

Meanwhile, in an email to Windows Azure CTP (Community Technology Preview) participants, Microsoft announced plans to transition Windows Azure from a CTP to a commercial offering by February 1st, 2010.

At PDC 2009, on November 17th, 2009, a number of new features in Windows Azure will be made available for the first time. The CTP will remain open through December 31st, allowing you to experiment with the full feature platform and to give us any final feedback.

Beginning January, 2010, new customers will have to sign up for an offer to access services on the Windows Azure platform. You’ll receive your first bill with a $0 balance, so you can see your exact usage while still enjoying free service.

On February 1, 2010, we will begin charging customers for using the Windows Azure platform.

I’ve been surprised how long Microsoft held off the official release of the commercial Azure platform, meanwhile loosing market share to Amazon and others. I’ll be interested to see what is released in November and how their pricing compares to Amazon.